Life Zone Ecology of the Bhutan Himaraya III

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Altitudinal patterns in clhimatologjcal, geomorphological, and ecological features of the Bhutanese life-zones were studied in a range of c. 1000-3500 m a.s.l. during 1999-2001. Temperature and moisture conditions were recorded in data loggers and obtained complete year round data from September 1999 to August 2001 along Tsang Chu. Temperature lapse rate was varied by the topographical situation of the measuring sites in a range of 0.57-0.62 C/100m. The lapse rate is larger in dry season (April) and smaller in monsoon rain season reflecting relative humidity. Mass movement is extremely active in the Bhutan Himalaya. Land slides took place by shearing on a bedding plane in the area underlain by metamorphic rocks. As the consequences river channels shift in the dip direction of the bedding plane and asymmetric valleys have developed. Vegetation zonation along altihidinal series were studied along Gasa series, Lobesa series, and Seagor-Lingmethang series. Gasa series mainly focused on topographical diversification within each life zone, Lobesa series had clear zonation from dry valley bottom to humid mountain cloud forests on ridges. From warm/dry Pinus roxburghii forests, mid-altitudinal Quercus griffithii/Q. lanata forests, Quercus glauca /Q. semecarpifolia forests, and Isuga dumosa forests and Abies densa forests above 2700 m. One third of the Bhutanese agricultural fields are maintained as shifting cultivation, and the sites are interesting for their secondary succession during fallow periods. There are two types of shifting cultivation in Bhutan, grass fallow (pangshing) of five year fallow and forest fallow (tseri) of 12 year fallow periods. For the former, we did in Bumthang, and for the latter in Shemgang, both place are the major area for both each types of shifting cultivation. In both sites we analyzed vegetation succession and soil developments, and found critical conditions for the sustainable managements for the system.